1. Field of the Invention
The present invention relates to the manufacture of semiconductor memory cells, more particularly, to a method of forming a trenched capacitor for a dynamic random access memory (DRAM) cell.
2. Description of the Related Art
In dynamic random access memories (DRAMs), information is typically stored by selectively charging or discharging each capacitor of an array of capacitors formed on the surface of a semiconductor substrate. Most often, a single bit of binary information is stored at each capacitor by associating a discharged capacitor state with a logical zero and a charged capacitor state with a logical one, or vice versa. The surface area of the electrodes of the memory capacitors determine the amount of charge that can be stored on each of the capacitors for a given operating voltage, for the electrode separation that can reliably be manufactured, and for the dielectric constant of the capacitor dielectric used between the electrodes of the charge storage capacitor. Read and write operations are performed in the memory by selectively coupling the charge storage capacitor to a bit line to either transfer charge to or from the charge storage capacitor. The selective coupling of the charge storage capacitor to the bit line is typically accomplished using a transfer field effect transistor (FET). The bit line contact is typically made to one of the source/drain electrodes of the transfer FET and the charge storage capacitor is typically formed in contact with the other of the source/drain electrodes of the transfer FET. Word line signals are supplied to the gate of the FET to connect one electrode of the charge storage capacitor through the transfer FET to the bit line contact facilitating the transfer of charge between the charge storage capacitor and the bit line.
There is a continuing trend toward increasing the storage density of integrated circuit memories to provide increased quantities of data storage on a single chip. To address the challenges of reduced structure sizes, DRAM designs have been proposed which incorporate capacitors having vertical extensions above the surface of the substrate (xe2x80x9cstackedxe2x80x9d capacitors) or below the surface of the substrate (xe2x80x9ctrenchedxe2x80x9d capacitors). By adopting a more three-dimensional structure, such DRAM designs provide memory capacitors having larger capacitance but which consume less of the substrate surface area.
FIGS. 1A to 1H, are cross-sections showing the manufacturing steps of fabricating a capacitor of a DRAM cell in accordance with the prior art.
Referring now to FIG. 1A, a semiconductor substrate 100, for example a p-type silicon substrate, is provided. A pad oxide 102, having a thickness of 400 angstroms, is grown on the upper surface of the semiconductor substrate 100 by thermal oxidation. A pad nitride 104, having a thickness of 1600 angstroms, is deposited on the pad oxide 102. Next, a silicon glass 108 such as boro-silicate glass (BSG), having a thickness of 5000 angstroms, is deposited by an in-situ doped chemical vapor deposition system. The silicon glass 108, pad nitride 104, and pad oxide 102 are defined by conventional photolithography so that an etching mask EM having an opening for deep trench is formed on the semiconductor substrate 100. Symbol 107 indicates a hard mask consisting of the pad nitride 104 and the pad oxide 102.
Referring now to FIG. 1B, the semiconductor substrate 100 is anisotropically etched through the opening of the etching mask EM to form a deep trench 112. Then, the silicon glass 108 is removed to expose the pad nitride 104 by an isotropic etching step. The deep trench 112 has a top portion TP, and a bottom portion BP.
As shown in FIG. 1C, a conductive layer 114, having a thickness of 50 to 400 angstroms, is deposited on the hard mask 107 and extended on the deep trench 112 by in-situ doped chemical vapor deposition. The conductive layer 114 can be a semiconductor material doped with arsenic ions.
Referring now to FIG. 1D, a photoresist 122 is formed in the bottom portion BP of the deep trench 112 to expose the conductive layer 114 at the top portion TP of the deep trench 112. Next, the exposed conductive layer 114 is removed followed by stripping the photoresist 122 as shown in FIG. 1E. An insulating layer 126 is formed on the pad nitride 104 and the deep trench 112 to cover the conductive layer 114. The arsenic ions formed in the conductive layer 114 are diffused and driven in the semiconductor substrate 100 so as to form a doped area having a depth of about 800 angstroms to serve as the lower electrode 130 of the capacitor. The insulating layer 126 is then removed.
As shown in FIG. 1F, a dielectric layer 132, for example silicon oxide/silicon nitride/silicon oxide (ONO), is formed on the conductive layer 114. Next, a doped polysilicon, to serve as the upper electrode 142 of the capacitor, is deposited on the dielectric layer 132 by low pressure chemical vapor deposition (LPCVD) followed by etching back. The dielectric layer 132 and the upper electrode 142 are also formed at the bottom portion BP of the deep trench 112.
Referring to FIG. 1G, a ringed insulating layer 160 is formed adjacent to the conductive layer 114 at a part of the top portion TP. Then, a conductive structure 164 whose upper surface is higher than that of the ringed insulating layer 160 is formed on the upper electrode 142 and the dielectric layer 132.
Next, a doped polysilicon film 166, having a thickness of about 600 angstroms, is deposited on the conductive structure 164 and the ringed insulating layer 160. The doped polysilicon film 166 has dopants such as phosphorus ions or arsenic ions therein. Next, an insulating layer 170, having a thickness of about 500 angstroms, is formed on the doped polysilicon film 166. The dopants of the doped polysilicon film 166 are then diffused and driven in the semiconductor substrate 100 at the top portion TP so as to form a doped region 172 to serve as the source region.
As shown in FIG. 1H, the hard mask 107 is removed. Shallow trench isolation 180 for defining an active region 186, gate oxides 184, and word lines 188 are then formed by conventional skills.
A higher level of integration, however, requires a proportional reduction in the area of the conducting elements of the capacitor in the DRAM cell, thus resulting in lower capacitance. The reliability of the data stored on the capacitor in the DRAM cell is therefore reduced. There is a need for providing a method of fabricating a capacitor, which has high capacitance.
In view of the above disadvantages, an object of the invention is to provide a method of fabricating a capacitor of a DRAM cell. According to the method, the lower electrode of the capacitor has rugged surface thus increasing the capacitance.
In accordance with an aspect of the invention, there is provided a method of fabricating a capacitor of a DRAM cell. First, a semiconductor substrate is provided. Then, an etching mask with an opening is formed on the semiconductor substrate. The semiconductor substrate is then etched through the opening of the etching mask to form a trench having a top portion and a bottom portion. Next, a ringed insulating layer is formed on the semiconductor substrate at the top portion of the trench. Afterward, a seed layer on the ringed insulating layer and the semiconductor substrate at the bottom portion of the trench. A photoresist is coated in the trench at the bottom portion. Next, the seed layer is partially removed to expose the ringed insulating layer while the photoresist is used as the shield. The photoresist is then removed to expose the remaining seed layer at the bottom portion. A hemispherical silicon grain layer is deposited from the remaining seed layer on the semiconductor substrate. Ions are doped and driven into the hemispherical silicon grain layer and the semiconductor substrate so as to create a doped area to serve as the lower electrode of the capacitor. Then, a dielectric layer is formed on the lower electrode followed by formation of an upper electrode on the dielectric layer.
In accordance with another aspect of the invention, there is provided a method of fabricating a capacitor of a DRAM cell. The semiconductor substrate can be a p-type silicon substrate. Furthermore, the ringed insulating layer can be a silicon oxide film having a thickness of about 200 angstroms to about 500 angstroms.
In accordance with another aspect of the invention, there is provided a method of fabricating a capacitor of a DRAM cell. The top portion has depth of about 1500 to 2500 angstroms form the upper surface of the semiconductor substrate.
In accordance with a further aspect of the invention, the seed layer is preferably an amorphous silicon layer that has a thickness of about 100 angstroms to 200 angstroms.
In accordance with a further aspect of the invention, there is provided a method of fabricating a capacitor of a DRAM cell. The formation of the ringed insulating layer can further comprise the steps of:
doping oxygen ions into the semiconductor substrate at the top portion of the trench at an angle of about 0 to 30 degrees with respect to a normal line of the upper surface of the semiconductor substrate while rotating the semiconductor substrate to improve doping uniformity throughout the semiconductor substrate at the top portion of the trench; and
annealing in argon ambient to form the ringed insulating layer.
Furthermore, in this method, the ion doping step can be carried out by phosphorous ions or arsenic ions at a dosage of about 1xc3x971019 atoms/cm2 to about 7xc3x971019 atoms/cm2.
In accordance with another aspect of the invention, there is provided a method of fabricating a capacitor of a DRAM cell. The formation of the etching mask further comprises the step of:
growing a pad oxide over the upper surface of the semiconductor substrate;
depositing a pad nitride on the pad oxide;
forming a silicon glass on the pad nitride; and selectively etching the silicon glass, the pad nitride and the pad oxide to form an etching mask with an opening.
In accordance with a further aspect of the invention, the silicon glass is preferably boro-silicate glass (BSG) deposited by an in-situ doped chemical vapor deposition system. The chemical vapor deposition system can be atmospheric pressure chemical vapor deposition (APCVD), sub-atmospheric chemical vapor deposition (SACVD), low pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD), or high density plasma chemical vapor deposition (HDPCVD.